The present invention concerns a cyclic process for producing an alkali solution of cellulose carbamate, for precipitating the cellulose carbamate from the alkali solution and then for recovering the remaining chemical for re-use in a recycling system.
Cellulose carbamate is an alkali-soluble cellulose derivative which is a reaction product of cellulose and isocyanic acid. It is possible to produce an alkali solution of cellulose carbamate which can be formed into fiber and film by using a precipitation bath which precipitates the cellulose carbamate from the solution.
For the precipitation bath a solution containing sulphuric acid has usually been used. Cellulose carbamate is stable in acid conditions and it is therefore not decomposed when thus precipitated. When an alkali solution, e.g., a sodium hydroxide solution, of cellulose carbamate is contacted with sulphuric acid, the cellulose carbamate is precipitated and at the same time, as the sodium hydroxide is neutralized, sodium sulphate is formed. In a continuous process both sulphuric acid and sodium hydroxide are consumed while, at the same time, more sodium sulphate is produced. Thus, the spinning process, if such is used, produces sodium sulphate, which must be removed from the process and sold as a by-product.
Another, more significant, drawback is that the sodium hydroxide neutralized by suphuric acid cannot easily be returned to the process. The sodium hydroxide accounts for a significant part of the raw material costs of the process. It would therefore be desirable to develop a precipitation method in which no undesired by-products are produced, a method which does not require use of a mineral acid and in which at least a substantial part of the sodium hydroxide can be recovered in an economical way.
At the same time, the method should meet certain requirements as regards the fiber or film quality. For instance, the initial strength of the fiber or film being precipitated should be sufficient to withstand mechanical strains to which it is subjected in the treatment. In fiber manufacturing, for instance, stretching the fiber is essential in the spinning step for achieving the desired strength properties.